This invention relates in general to testing apparatus for electronic devices. More specifically it relates to a contactor assembly that is generally frequency insensitive to allow broad band testing with fast-rising signals.
In the manufacture and use of integrated circuits (IC's) and similar electronic devices it is important to test the devices accurately, reliably and at a high rate. Automatic testing and handling apparatus machines that can perform this task are available. Such apparatus suitable for testing dual-in-line packaged (DIP) IC's are sold by the Daymarc Corporation, Waltham, Mass., under the trade designation Type 1156 and 1157. In a DIP device, the circuit is contained in a molded plastic body having a generally rectanguar, box-like configuration. Two rows of generally parallel connecting pins are arrayed along parallel sides of the body with each pin extending in a direction generally normal to the main faces of the body.
In each of the aforementioned apparatuses the IC's are momentarily brought to rest at a test station where a set of contacts, typically double Kelvin contacts, are flexed through a cam action into electrical connection with the pins of the device. The contacts establish an electrical connection between testing circuitry and the device. The contacts are usually part of a probe or contactor assembly which includes an insulating base member that mounts the contacts. The contacts are typically narrow strips of a resilient and highly conductive material. The contacts typically make electrical connection with an associated connecting pin at a free end opposite the base. The cross-sectional dimensions of the contacts are relatively small due to (1) the requirement that all of the contacts simultaneously make connection a set of closely packed pins and (2) the requirement that the contacts flex for millions of cycles of operation without material fatigue. The length of the contacts is determined by the spacing between the test station of the IC handling apparatus and the test circuitry.
Frequently the testing of the integrated circuits requires that the testing signal be "fast-rising", that is, a signal which is a very steep, step-like increase in potential. A typical fast-rising signal is characterized by a voltage change of 5 volts per nanosecond. Such a signal can be represented through Fourier analysis as being composed of a multitude of superimposed sine waves having a very high frequency, typically on the order of 300 mHz. The fast-rising signal launched by the test circuitry and carried by the contacts to the device therefore contains components with a very high frequencies.
A major problem with this testing arrangement is that due to the inherent inductance of the contacts themselves, the signal encounters an inductive reactance X.sub.L. This reactance produces distortions and reflections which degrade the quality and accuracy of the test. The inductance L of the contact is a function of the cross-sectional area of the conductor and its length. It increases directly with the length and inversely with the cross-sectional area. Since the inductive reactance X.sub.L =2.pi.fL, for the very high frequencies f associated with a fast-rising signal, the inductive reactance associated with even the relatively short contacts in normal use becomes a significant source of distortion and limits the accuracy of measurements.
One possible solution would be to increase the cross-sectional area of the contacts. However, the physical constraints of the test environment limit the useful dimensions of the contacts. For example, the contacts must be separated laterally from adjacent contacts while still maintaining a unique association with one pin on the IC. Also, the contacts must be sufficiently thin to flex repeatedly without exhibiting fatigue. Another possible solution is to make the contacts shorter. This solution works well if the IC can be placed manually into the test circuit. However, with high speed automated operation (e.g. 6,000 units per hour), the test circuitry must be physically separated from the device handling mechanisms with electrical connection made over some short distance spanned by a probe or contactor assembly of the type described above. In short, modern production economics require contacts having a length which is troublesome for fast-rising signals. Another solution is surrounding each contact with a shield in the manner of a coaxial cable. The shield, however, would interfere with the flexure of the enclosed contact. Still another possible solution is simply to test each device more slowly to wait for distortions and reflections to die out. With many modern IC's however, the speed of operation of the device itself is so fast that if the testing operation were to extend over a sufficient period of time to allow distortions and echoes induced by the fast-rising testing signal to subside, then the speed rating of the devices cannot be determined. In short, the testing operation must have a speed comparable to that of the device function being tested. At present, there is no known contactor assembly for use with automated IC testing and handling apparatus which can provide a reliable electrical connection between the IC device and the testing circuitry while at the same time avoiding the distortions, reflections and the resulting uncertainty of the measurement when the IC is tested with fast-rising signals.
Another consideration is minimizing "ground noise", that is, changes in the reference voltage due to current surges during the test procedure simulating operation of the device. A typical situation is a test where a change in the device state causes a current surge in the range of 20 milliamperes per nanosecond. Such a surge can cause the ground reference to move 1 volt or more thereby distorting measurements referenced to ground by 20% or more. The end result is that good devices may not pass the test and are downgraded.
It is therefore a principal object of this invention to provide a contactor assembly for testing electronic devices, particularly high speed IC's that presents substantially no inductive reactance to a fast-rising signal launched in any contact of the assembly.
Another object of the invention is to provide a contactor assembly with the foregoing advantage which is characterized by low signal distortion and reflection and a substantially resistive or "characteristic" impedance.
Yet another object of the invention is to provide a contactor assembly with the foregoing advantages which allows testing at high production speeds and with extremely high degrees of accuracy and hence certainty.
A further object of the invention is to provide a contactor assembly that can produce a test current surge originating very close to the device, presents a low inductance path for the surge to the device, and substantially reduces ground noise normally attendant such a surge.
A still further object of the invention is to provide a contactor assembly with the foregoing advantages that has a generally simple, low cost and highly durable construction.